The present invention relates generally to calcination and, more particularly, to indirectly heated calcination. Calcination is the process of subjecting a material to prolonged heating at fairly high temperatures.
In directly heated calcination, the material to be calcined is exposed to the source of heat, for example, the calcination of Al(OH)3 to Al2O3 in which Al(OH)3 is directly heated by combustion of oil, gas or coal. In indirectly heated calcination, the material to be calcined is isolated from the source of heat. Typically the material to be calcined is contained within a cylindrical retort which is rotated within a stationary refractory lined cylindrical furnace with combustion of fuel occurring within the annular ring between the retort and the furnace. Such calciners have been used for activating wood charcoal, reducing mineral high oxides to low oxides, drying fluoride precipitates in a hydrogen fluoride atmosphere, calcination of silica gel, drying and removal of sulphur from cobalt, copper and nickel, reduction of metal oxides, oxidising of organic impurities, and reclamation of foundry sand.
The present invention is not concerned with directly heated calcination and, in contrast to known indirectly heated calcination processes, the present invention is concerned with indirectly heated calcination in which the material to be calcined is contained within a calcination vessel and is heated by heat transferred from a liquid metal flowing through a heat exchanger within the calcination vessel. The present invention is applicable to both processes in which a fluidised bed of the material to be calcined is formed within the calcination vessel and to processes in which fluidisation is not utilised, for example, indirectly heated rotating drum calcination. The present invention is also applicable to processes in which the pressure within the calcination vessel is atmospheric, greater than atmospheric, or less than atmospheric.
In a first aspect, the present invention provides an apparatus for calcining a material, the apparatus comprising a calcination vessel which houses a heat exchanger arranged to transfer heat to the material from a liquid metal heat exchange fluid arranged to flow through the heat exchanger.
The apparatus according to the first aspect of the present invention may comprise a single calcination vessel. Alternatively, the apparatus may comprise a series of calcination vessels, each of the series of calcination vessels being arranged to partially calcine the material. Typically, the series of calcination vessels will comprise two or three calcination vessels.
In a second aspect, the present invention provides a process for calcining a material in an apparatus according to the first aspect of the present invention, the process comprising transferring heat to the material from a liquid metal heat exchange fluid flowing through the heat exchanger within the calcination vessel.
Where the apparatus according to the first aspect of the present invention comprises a single calcination vessel, the process according to the second aspect of the present invention is a single-stage calcination process and where the apparatus comprises a series of calcination vessels, the process is a multi-stage calcination process.
In a third aspect, the present invention provides material calcined by a process according to the second aspect of the present invention.
Liquid metals suitable for use in the present invention are characterised by having relatively low melting points, relatively high boiling points, relatively high heat transfer coefficients, relatively high specific heats and relatively low viscosities. Such liquid metals include sodium, potassium, magnesium, lead, tin, mercury and alloys thereof. A sodium-potassium alloy comprising 22% by weight sodium and 78% by weight potassium is an example of a suitable liquid metal alloy.
The heat exchanger(s) housed within the calcination vessel(s) may form part of a closed loop with the liquid metal heated externally of the calcination vessel(s) by heating means.
Liquid metals such as sodium and potassium are very reactive and hence for safety reasons the liquid metal is isolated from the atmosphere and other sources of reactant. The liquid metal may therefore be caused to flow through the heat exchange loop by use of one or more mechanical or electromagnetic pumps.
Liquid metals are electrical conductors and hence can be forced to flow under the influence of a magnetic field when a current is passed through the liquid metal normal to the direction of the magnetic field. Force is exerted on the liquid metal in a direction normal to both the magnetic field and the current flow. For example, a portion of the heat exchange loop may be passed horizontally between the poles of an electromagnet (arranged to impart a vertically orientated magnetic field) with an externally sourced current passed horizontally across the liquid metal in the magnetic field in a direction normal to the desired direction of flow of the liquid metal. Electromagnetic pumps are advantageous because they do not have any moving parts.
Preferably, the liquid metal is caused to flow through the heat exchange loop by being passed through one or more centrifugal pumps. Centrifugal pumps are preferred to electromagnetic pumps because centrifugal pumps are more efficient and are capable of pumping greater volumes. However, high operating temperatures necessitate careful design of centrifugal pumps used for pumping liquid metal. Factors to be considered in the design of a centrifugal pump for pumping liquid metal include the dissipation of heat from the pump, the expansion of components of the pump, the critical speed of rotation of the shaft, operation of the bearing in liquid metal, and sealing of the shaft to prevent leakage of liquid metal.
In either case, it is preferred that the pump or pumps are located at the coolest point in the heat exchange loop, for example, between the exit of the heat exchanger of a single-stage calcination process and the point where the liquid metal is heated.
As an alternative to a pump, a thermosiphon may be used to induce flow of a liquid metal through the heat exchange loop. Thermosiphon circulation can be induced provided that there is a sufficient difference in density between the hot and cool portions of the liquid metal.
The heat exchanger may take a variety of forms. The heat exchanger may simply be a pipe passing through the calcination vessel. However, to increase the transfer of heat to the material within the calcination vessel, it is preferred that the heat exchanger is arranged to maximise the surface area for heat transfer. The heat exchanger may take the form of a pipe or pipes having a serpentine passage through the calcination vessel. Alternatively, the heat exchanger may take the form of a series of pipes which are connected by manifolds or pigtails.
As an alternative to a heat exchange loop through which the liquid metal is pumped, the liquid metal may be contained within one or more heat pipes. Each pipe is part of a closed, normally evacuated, system which protrudes within the calcination vessel as the heat exchanger. Heat is supplied to a portion of the system external to the calcination vessel. For example, a heat pipe may take the form of a connecting pipe passing through the bottom of the calcination vessel which joins a base portion to a heat exchange portion. Heat may be applied to the base portion from an external source, for example, by combustion of gas or the like, resulting in heating of the contained liquid metal so as to generate a vapour and passage of the metal vapour through the connecting pipe to the heat exchange portion where the vapour condenses on the walls of the heat exchange portion with heat transferred to the material within the calcination vessel. On cooling, the liquid metal in the heat exchange portion returns through the connecting pipe to the base portion where it is again heated to vapour. A convection flow of liquid metal and vapour is thus created in the heat pipe with the heat from the vapour being transferred to the walls of the heat exchange portion and subsequently to the material in the calcination vessel. Heat pipes are advantageous because no pumping of the liquid metal is required.
One application of the present invention is the calcination of magnesium chloride hexammoniate (MgCl2.6NH3) to anhydrous magnesium chloride (MgCl2). The present invention will hereafter be described in relation to such application but it is to be expressly understood that the present invention is not restricted to such application.
Magnesium metal can be electrolytically produced from MgCl2 and MgCl2 can be produced by calcination of MgCl2.6NH3 with liberation of ammonia (NH3). Calcination of MgCl2.6NH3 for subsequent production of magnesium metal is problematic for a number of reasons.
A large quantity of heat is required because MgCl2.6NH3 must be calcined at high temperature, for example, in the order of 480xc2x0 C. to produce MgCl2.
Directly heated calcination is not feasible because of the level of purity required of the product MgCl2.
Commercial production of magnesium metal by electrolysis of MgCl2 requires the calcination of large quantities of MgCl2.6NH3.
The calcination environment is corrosive and hence the calcination vessel must be manufactured from expensive materials to limit contamination of the product MgCl2.
The calcination process is a pressurised process.
Long residence times in the calcination vessel are undesirable due to the increased likelihood of contamination.
For MgCl2 to be produced from MgCl2.6NH3, 6 molecules of NH3 must be removed from each molecule of MgCl2.6NH3. In a single-stage calcination process according to the second aspect of the present invention, the calcination reaction is:
MgCl2.6NH3xe2x86x92MgCl2+6NH3
with sufficient energy being required within the calcination vessel to remove all 6 molecules of NH3.
A multi-stage calcination process according to the second aspect of the present invention is advantageous because overall less energy and heat exchange area is required. By way of example, a two-stage calcination process may be represented as
MgCl2.6NH3xe2x86x92MgCl2.2NH3+4NH3xe2x80x83xe2x80x83Stage One
MgCl2.2NH3xe2x86x92MgCl2+2NH3xe2x80x83xe2x80x83Stage Two
and a three-stage calcination process may be represented as
MgCl2.6NH3xe2x86x92MgCl2.2NH3+4NH3xe2x80x83xe2x80x83Stage One
MgCl2.2NH3xe2x86x92MgCl2.NH3+NH3xe2x80x83xe2x80x83Stage Two
MgCl2.NH3xe2x86x92MgCl2+NH3.xe2x80x83xe2x80x83Stage Three
MgCl2 is preferably produced from MgCl2.6NH3 in a single-stage or multi-stage fluidised bed calcination process with NH3 utilised as a fluidising gas. High purity of the product MgCl2 is highly desirable because the presence of contaminants can adversely affect the electrolytic production of magnesium metal from MgCl2. It is therefore preferred that at least the interior of the calcination vessel(s) and the exterior of the heat exchanger(s) are manufactured from a material which will introduce a minimum of contaminants and which will resist deterioration. Stainless steel is preferably not used because of the possibility of loss of metal or deterioration in its properties at operating temperatures. It is therefore preferred to use special alloys such as INCONEL 600 or INCONEL 601 which exhibit high corrosion resistance, strength and stability at high temperature. Alternatively, the calcination vessel(s) may be manufactured from carbon steel and internally lined with insulating ceramic bricks or refractory.
As previously mentioned, the heat exchanger(s), housed within the calcination vessel(s) may form part of a closed loop with the liquid metal heated externally of the calcination vessel(s) by heating means. In such a case it is preferred to manufacture those portions of the heat exchange loop external to the calcination vessel(s) from a material such as stainless steel to minimise costs. The liquid metal may be heated by routing the heat exchange loop through heating means in the form of one or more hydrocarbon fuel fired boilers or electric heaters. The temperature within the calcination vessel of a single-stage calcination process or within the final calcination vessel of a multi-stage calcination process is preferably in the range 460-500xc2x0 C., more preferably about 480xc2x0 C. For a single-stage calcination process the liquid metal preferably enters the heat exchanger at a temperature in the order of 700xc2x0 C. and exits the heat exchanger at a temperature in the order of 550xc2x0 C., whereafter it is heated to approximately 700xc2x0 C. prior to again entering the heat exchanger.
For a two-stage calcination process the liquid metal preferably enters the heat exchanger of the first calcination vessel at a temperature in the order of 550xc2x0 C. and exits at a temperature in the order of 300xc2x0 C. which is believed to provide a temperature within the first calcination vessel in the range of 210-230xc2x0, preferably about 220xc2x0 C. A calcination temperature of approximately 220xc2x0 C. is believed to be sufficient to remove 4 molecules of NH3 from a molecule of MgCl2.6NH3 in accordance with the following reaction:
MgCl2.6NH3xe2x86x92MgCl2.2NH3+4NH3.
The liquid metal is preferably heated to approximately 700xc2x0 C. between the first and second calcination vessels prior to entering the heat exchanger of the second calcination vessel where the final two molecules of NH3 are believed to be removed in accordance with the following reaction:
MgCl2.2NH3xe2x86x92MgCl2+2NH3.
The liquid metal preferably exits the heat exchanger of the second calcination vessel at approximately 550xc2x0 C. whereafter it is preferably returned to the heat exchanger of the first calcination vessel for the heating cycle to be repeated.
In the two-stage calcination process referred to above, preferably, MgCl2 is continuously withdrawn from the second calcination vessel, MgCl2.6NH3 is continuously introduced into the first calcination vessel, and MgCl2.nNH3 (where n is approximately 2) is continuously transferred from the first calcination vessel to the second calcination vessel.
In a similar manner to that described above for a two-stage calcination process, MgCl2.6NH3 may be calcined to MgCl2 in a three-stage calcination process in accordance with the following reactions:
MgCl2.6NH3xe2x86x92MgCl2.2NH3+4NH3xe2x86x92MgCl2.NH3+NH3xe2x86x92MgCl2+NH3.
Multi-stage calcination is preferred to single-stage calcination because:
(a) the cumulative surface area of the heat exchangers can be reduced as compared with the surface area of a single-stage heat exchanger;
(b) lower liquid metal exit temperature from the heat exchanger of the first calcination vessel as compared with the liquid metal exit temperature from the heat exchanger of a single calcination vessel enables pumping, flow measuring and controlling of the liquid metal to be conducted at lower temperature, thus allowing less expensive equipment to be used;
(c) in multi-stage calcination, a larger overall temperature difference of the circulating liquid metal enables a smaller capacity pump to be used; and
(d) multi-stage calcination enables higher efficiency in the heating of the liquid metal.